Usually when a preformed pond springs a leak, it is where the pond has ledges. We tend to step on the ledge when we get in the pond and because we have not seated it properly in the ground (almost an impossibility), it cracks. Now what?
I know of no way to patch a preformed hard plastic pond. I have heard of folks using Bondo, but have not seen it first hand. Because they are impossible to patch and crack so readily, I recommend against using them. If you wish to use a preformed pond, go to a feed store and buy a horse feeding trough. Rubbermaid makes them, they have a flat bottom, no ledges, are virtually indestructible and are much much cheaper than the preformed pond you find at the big box store.